Typical IC packages usually include an enclosed and/or encapsulated plastic housing with an internal lead frame, bond wires and die, as well as external electrical leads for electrically connecting the die to the outside world. IC packages that house higher thermal dissipation die have an additional integral heat sink formed within the package that is connected to the IC substrate. The heat sink typically comprises a metal layer. A die is usually mounted on this metal layer with its electrical contact pads face up and bond wires connecting the electrical contact pads of the die to the internal lead frame of the IC package. The lead frame provides a mechanically rigid electrical path from the die and the bond wires to the external electrical leads of the IC package. The lead frame is usually molded partially within the plastic housing in order to bridge the electrical connection between the internal lead frame and the external leads. The electrical leads make the electrical connection from the package to a circuit board (CB) on which the package is mounted.
When a high-power thermal dissipation IC is mounted on a CB, a heat spreader device on the CB located directly underneath the IC package is thermally connected to the heat sink of the IC package and then to a ground plane within the layers of the CB, which is typically a printed circuit board (PCB) layers. The heat spreader device dissipates heat generated by the IC into the CB ground plane itself. Heat generated by the IC is also convectively moved away from the IC package out to the surrounding air. However, when the use of the IC package is constrained to a small physical location and/or is physically small relative to other die, there is usually very poor circulation of air around the package, and thus very little removal of heat by convection. All of the heat generated by the IC must then be mostly dissipated in a direction down through the IC package heat sink and into the heat spreader device of the CB.
In direct-chip-attach configurations, the IC package is eliminated and the die is mounted directly to the CB in an inverted position such that the electrical contact pads on the die face downward toward the confronting circuit traces on the CB, and are electrically connected by electrical interconnects (e.g., solder bumps) that connect the contact pads of the die to the circuit traces on the CB. The CB used in a direct-chip-attach configuration is usually a physically flexible PCB and is known as a flex CB or flex circuit. Because of its flexibility, the flex CB can be shaped into a small physical area, while still maintaining the electrical contact between the CB and the die. These types of circuits are often used in situations where very little space is available to mount the CB, such as on a read/write head that reads data from and writes data to a hard drive magnetic recording media within a hard disc drive (HDD). These read/write heads typically sit on a stainless steel armature with the flex CB mounted upon it. The electrical contacts of the flex CB typically connect the read/write heads to electrical contacts of a read/write preamplifier IC. This entire assembly is then floated aerodynamically above the magnetic recording media.
FIG. 1 illustrates a perspective view of a typical direct-chip-attach assembly 11, which includes a flex CB comprising various layers 12 and an IC die 13 mounted on the flex CB 12. The IC die 13 is mounted to the flex CB 12 in a confronting position such that electrical contact pads (not shown) on the die 13 are disposed to be easily connected by electrical interconnects (e.g., solder bumps) 19 to the circuit traces (not shown) on the flex CB 12. The flex CB 12 may include a heat sink material 14 that is in contact with the side of the CB 12 that is opposite the die 13, a layer 15 of adhesive and thermally insulating dielectric material (e.g., polyimid) disposed on the heat sink material 14, a metal layer 16 disposed on layer 15, and a layer 17 of thermally insulating dielectric material (e.g., polyimid) disposed on the metal layer 16. The aforementioned circuit traces are formed by portions of the metal layer 16 of the flex CB 12, which is typically made of copper. The heat sink material 14 functions as the heat dissipator and the thermal path is in the direction from the die 13 to the heat sink material 14, as indicated by the arrows 23 and 24 directed from the die 13 into the heat sink material 14.
In some cases, flex CBs include a stabilizing device, such as an aluminum stiffener (not shown), that is located between the heat sink material 14 and the layer of polyimid and adhesive 15. The stiffener provides mechanical stability to the flex CB. In flex CBs, the stiffener may function as both a heat sink and as a stabilizing device, in which case the heat sink material 14 may be omitted. The heat sink material 14 need not be part of the CB 12, but may instead be a separate device upon which the CB 12 is placed. If a stiffener is used, it provides a path of heat dissipation into the heat sink material 14 and ultimately into the armature and housing of the disc drive.
Typically, the electrical interconnects 19 that connect the contact pads of the die 13 with the circuit traces formed in the metal layer 16 are solder or lead-free bumps that are placed on the electrical contact pads (not shown) located on the bottom face of the die 13 and heated and then placed in contact with the circuit traces on the flex CB 12. When the bumps cool and harden, they form a rigid electrical connection between the pads on the bottom face of the die 13 and the circuit traces formed in the metal layer 16 of the flex CB 12.
Once the electrical connections have been made between the pads on the die 13 and the circuit traces of the flex CB 12, a slight separation exists between the surface of the die 13 and the surface of the CB 12. Due to the physical geometry of this spacing, which is typically in the range of 25 to 76 micrometers (0.001 to 0.003 inches), the spacing between the die 13 and the flex CB 12 typically is filled with an underfill material 21 that provides mechanical stability. This is intended to prevent undue mechanical stresses from being exerted upon the die 13 and the interconnects that could cause the electrical connections to fail. The underfill material 21 is usually applied after the pads of the die 13 have been interconnected with the circuit traces on the flex CB 12. The underfill material 21 is typically applied using capillary flow. The underfill material 21 is then heated in order to cure the material into a solid, physical state. The underfill material 21 that is currently used for this purpose has poor thermal conductivity and is typically Hysol®FP4549, manufactured by the Henkel Loctite Corporation of Dusseldorf, Germany. This particular underfill material is a high purity, low stress, liquid epoxy designed for enahanced adhesion to integrated circuit passivation materials.
When a flex CB assembly such as that shown in FIG. 1 is used on a read/write head of a disc drive, the flex CB assembly normally uses large amounts of current and/or voltage to enable the read and write operations to be performed. These types of signals typically exhibit very fast rise times, some less than 200 picoseconds (ps), and large slew rates in excess of 700 miliamperes (mA) per nanosecond (ns), which produce extremely large instantaneous currents and/or voltages. These large instantaneous currents and/or voltages produce a large amount of thermal energy that needs to be dissipated.
Several attempts have been made to improve the effectiveness of the heat sink of the CB assemblies, including increasing the copper trace area on the flex circuit, increasing the copper trace thickness on the flex circuit, using higher density thermal conductivity interconnects such as dedicated locations of multiple “dummy” bumps, using higher thermal conductivity underfill, and adding heat sinks to the side of the flip-chip opposite the CB to help improve convective cooling into the surrounding air in addition to the conductive cooling already occurring through the physical structure of the die/CB interface. To date, none of these techniques, used either individually or together, have proven completely effective at significantly reducing the thermal resistance while also providing an effective low cost (or free) solution.
When a flex CB assembly is used in a very physically small environment, such as on a read/write head of a disk drive, for example, where space and cost constraints are at a premium, typical approaches for reducing thermal resistance are inadequate and/or impractical. In addition, flex CB assemblies typically use a single-layer (i.e., the metal layer 16 having traces formed in it). In cases where it is possible and practical to use a multi-layer CB, such as where space and cost constraints are not an issue, simple multi-layer plated through-hole technology can be used to provide thermally conductive heat paths down through the CB in order to dissipate thermal heat generated. However, multi-layer CBs usually cost considerably more than single-layer CBs. Therefore, using a multi-layer conductor CB may be cost prohibitive in some cases. Also, due to the aerodynamics of the head armature in a disc drive application, multi-layer CBs located on the armature of a disc drive are usually unsuitable because the additional mass on the armature can result in slower read and write speeds.
Accordingly, a need exists for a method and apparatus for more effectively dissipating thermal energy in CB assembly, particularly in a direct-chip-attach assembly.